The sessile bivalve mussels belonging to the Anomiidae attach to substrates through a highly specialized byssus that extends from the animal through a notch in one of the shells. In most members of the family, the byssus is heavily mineralized. We review the design and formation of the mineralized byssus in the best studied member of the family: the jingle shell Anomia simplex. This byssus contains over 90 wt% CaCO3 found as a mixture of the crystal forms aragonite and calcite. Magnesium can substitute calcium in calcite. To enhance our understanding of the byssus design, we determine the distribution of magnesium substitution by a combination of x‐ray diffraction and energy dispersive X‐ray spectroscopy. Combined with the literature data, the present work displays the highly complex hierarchical design of the unique attachment system of Anomia.